Fusobacteria have been implicated in periodontal diseases and have been isolated from brain and pulmonary abscesses. We have observed hemagglutination of human and animal red blood cells with an antigen preparation of these organisms. This hemagglutination of red blood cells may be useful in the taxonomic separation of this group of microorganisms. Sera and saliva have shown inhibition of the hemagglutination in a hemagglutination inhibition test and the test may be useful in studying the immune response to these organisms. The receptor(s) responsible for the hemagglutination may be valuable in the colonization of the microorganism in the oral cavity and the inhibition of colonization by saliva or sera may be an important immune mechanism in antigen deletion. Studies include characterization of the hemagglutinating moiety, the types of human cells it hemagglutinates, the identity of the receptor on the human cells, and the substance in saliva and serum which inhibits the hemagglutination. BIBLIOGRAPHIC REFERENCE: Falkler, W.A. and C.E. Hawley, 1977. Hemagglutinating Activity of Fusobacterium nucleatum. Infection and Immunity 15:230-238.